Abstract

A diverse set of mesoscale observations collected in the National Hail Research Experiment in connection with an evolving Colorado hailstorm is analyzed to determine the kinematic and thermodynamic structure of the near environmental and subcloud regimes. The analysis centers on multi–level aircraft measurements in the inflow sector and on mesoscale observations at the surface and aloft. Although considerable evolution was observed in overall radar echo development patterns, changes in one of the many storms detected occurred in a systematic and periodic manner. The instantaneous structure of the subcloud inflow into this storm is emphasized in the present work.

Surface and aircraft data substantiate the following characteristics. Inflow air approached the front of the storm, originating from a very shallow layer (≤5 m) near the ground and at a considerable distance (≥20 km) upstream in the relative wind direction. Inflow air rose unmixed to at least cloud base, feeding the main updraft which was inclined upward in a direction opposite to the storm movement. Discrete inflow–updraft branches were found to be supporting coexisting cells in varying stages of development. These had lateral widths of 6–8 km and were separated by regions of weak subsidence. Downdraft air approached the storm from the right flank at mid-cloud level and at least a portion descended unmixed the ground in the strongest downdrafts.

Abstract

Guidelines followed in designing and operating a special mesoscale rawinsonde network are discussed. Objective data reduction and analysis techniques are developed and applied to the wind, temperature and moisture data measured during a selected thunderstorm case. The goal is to appraise the sounding system's limitations and reliability for resolving the mesoscale circulations associated with convective processes. A consistent four-dimensional synoptic portrayal of the variables is achieved by accounting for balloon drift, differing station-to-station and sounding-to-sounding ascent rates, and departures from scheduled release time.Temporal variations in the spatial distributions of computed divergence and kinematic vertical motion are in good qualitative agreement with the location and intensity of thunderstorm radar echoes, after further objective adjustments are applied to compensate for the assumed character of wind measurement and analysis errors. For the purpose of assessing data and analytical credibility, independent estimates of the energy related to diabatic heating are obtained by evaluating thermodynamic energy and moisture continuity equations. Comparative results indicate that the network upper-air data and the methods adopted for its synthesis are internally consistent. On the other hand, the distribution and magnitude of the resolved kimematic and dynamic features demonstrate that the results are clearly a function of observational spacing and do not necessarily pertain to individual convective processes.

Abstract

The precipitation efficiency (the ratio of surface rainfall to water vapor inflow) of a small sample of thunderstorms observed in the Cooperative Convective Precipitation Experiment is calculated using surface and cloud-base airflow and moisture measurements and subcloud rainout based on radar reflectivity factor. Highly-resolved vertical flux measurements from aircraft indicate that a significant amount of water vapor inflow may have been overlooked in past work of this kind, resulting in overestimates in precipitation efficiency. Trends in the mass of water vapor influx resolved at intervals of 10 to 30 min are corroborated by the evolution of water vapor flux convergence computed at 5-min intervals from objectively analyzed surface mesonetwork observations.

Fluxes of water vapor inflow and precipitated rainwater are integrated over periods exceeding an hour to obtain precipitation efficiencies applicable to the mature storm phase. Precipitated rainwater estimates from radar reflectivity-rainrate relation-ship suffer from the usual uncertainties involved in single-parameter radar rainfall estimation, but neither the choice of a particular Z-R relation nor the upper reflectivity threshold assigned to compensate for the likely presence of hail changed the order of the storms when they were ranked according to their precipitation efficiency. This permits a meaningful comparison with environmental factors even though the absolute accuracy of the precipitation efficiencies is somewhat in doubt. Results indicate that factors controlling thunderstorm precipitation efficiency are more complicated than a simple inverse dependence on vertical wind shear, as advanced in earlier work, and that other environmental parameters undoubtedly come into play. In the present analyses, for example, subcloud mixing ratio, shear kinetic energy in the lower troposphere and cloud base area all exhibit weak positive correlations with precipitation efficiency, while there was a tendency for storms with high bases to display a slight inverse correlation.

Abstract

With an attainable height resolution of no better than ±10 m from individual soundings, direct analysis of geopotential height gradient from mesoscale rawinsonde ascents could not be expected to produce reliable results. A method is presented for deriving a height field which incorporates the carefully analyzed horizontal winds measured by an upper air network specifically designed to study severe thunderstorms. Three-dimensional distributions of wind, temperature and moisture content are obtained as a function of time using a combination of subjective and objective analysis techniques, which take account of departure from scheduled release time, differing ascent rates, and the horizontal drift of balloons during ascent.

The vertical component of air motion is computed from the kinematic approach. Adjustments are applied to divergence estimates to achieve physically realistic results for vertical motion in the upper troposphere. Errors in the horizontal wind components are likewise altered for consistency by assuming that they affect only the divergent part of the flow. The three-dimensionally consistent array of velocity components is used to evaluate the complete horizontal divergence equation to obtain the geopotential Laplacian as a residual, which, when integrated numerically, yields the horizontal height perturbations associated with the mesoscale winds. Results obtained from application of these techniques to rawinsonde data collected in a squall line case appear to be in qualitative agreement with recognized thunderstorm airflow and pressure distributions. Comparative magnitudes of individual terms in the divergence equation demonstrate that the balance approximation is inadequate for diagnosing the dynamics of mesoscale convective motions.

Abstract

Observations from aircraft, Doppler radars, surface mesonetwork, upper air network and surveillance radar are used to describe the structure and evolution of a large rain and hailstorm that occurred on 22 June 1976 in northeastern Colorado. In its early stage, the storm was unicellular in nature and was moving northward somewhat slower than the middle-level environmental winds. Once significant outflow developed beneath the storm, new cells started their growth over the southeastwardly progressing outflow boundary and then advected northward and interacted with the now nearly motionless main updraft. For about 45 min the storm exhibited characteristics of the steady, vaulted supercell, though overall it was still clearly multicellular in nature. During this stage nearly 2.5×10⁹ kg s⁻¹ of air and 2×10⁷ kg s⁻¹ of moisture were being processed through cloud base within the vault updraft. Precipitation that grew to millimeter size within the small (about 5 km diameter) cells served as embryos for continued growth to hailstones as big as 3 cm diameter within the main updraft associated with the weak echo vault. Calculated ice particle growth trajectories in the nearly steady phase indicated that recycling of precipitation particles within a single updraft was not possible, however, as all particles followed simple paths northward across and to the sides of the main updraft core, failing out as rain, graupel or hail on the north and west sides of the updraft. Small particles that entered the main updraft where speeds exceeded 15 m s⁻¹ were exhausted into the anvil as 5–8 mm graupel.

Abstract

A case study is presented of a persistent thunderstorm of moderate intensity which occurred in northeast Colorado, and which produced a light hailfall at the ground. The storm was intensively monitored by aircraft, radar, dropsondes, and surface and upper air networks involved in the National Hail Research Experiment. The present study emphasizes the measurements obtained by instrumented aircraft in examining the properties of the subcloud airflow.

Large-scale analysis shows that the storm formed on a surface confluence line and subsequently moved toward the region of surface moisture. A brief radar analysis of the storm during the mature stage of its ∼5-hr lifetime is presented, and identifies the general features as belonging to the category termed “supercell” by previous workers. A precipitation analysis is carried out, and rain and hailfall are correlated with the track of the storm.

Liberal use of a time-space conversion technique results in detailed mesoscale pressure, temperature, moisture and wind field analyses from a network of 22 remote meteorological stations. Surface divergence of mass and moisture is computed. Surface features are related to the position and structure of the radar echo.

Wind, temperature and moisture data obtained by aircraft encircling the storm in the region below cloud base are presented. Emphasis is placed on the airflow with respect to the moving storm, and details of the subcloud circulation are examined. Analysis of relative streamlines, supported by the observed temperature and moisture structure, delineates distinct regions of inflow and outflow for the storm. Partitioning of the measured flux into inflow and outflow segments at the aircraft levels and at the surface results in estimates of the mass and moisture budgets for the storm. The ratio of rainout measured at the ground, 2 × 10⁹ gm sec⁻¹, to the computed moisture influx, 13 × 10⁹ gm sec⁻¹, results in a precipitation efficiency of only 15%. A physical basis (following a discussion presented by Marwitz) for correlating this low efficiency with the fairly high value of vertical wind shear which existed around the storm, ∼5 × 10⁻² sec⁻¹, is elaborated upon, and the present results are compared with those of previous investigators. The computed mass inflow, 2 × 10¹² gm sec⁻¹, is rather large, but is shown to be compatible with other measurements made of the storm. The computations indicate that, in this case, only about half of the upward flux took place in the vicinity of the “echo-free vault,” and attention is drawn to a secondary region of echo overhang where various measurements indicate that significant vertical transports also occurred. Based on the airflow and thermodynamic measurements, some ideas concerning the energy source that drives the inflow circulation are presented.

Abstract

Data from five Doppler radars, the surface mesonet, aircraft, and rawinsondes from the Cooperative Convective Precipitation Experiment (CCOPE) are used to document the structure and evolution of a squall line with unusually persistent cells and an anvil that spreads downwind in strong upper-level westerlies. The environmental sounding showed linear shear of ∼4 m s⁻¹ km⁻¹ through the troposphere, a convective available potential energy of 600 m² s⁻², and a convective Richardson number of 10, based on the wind in the lowest 6 km.

The orientation of the squall line, comprised of high-reflectivity centers spaced 20–40 km apart, changed with time. Initially, the squall-line axis was normal to the environmental shear, but with time it became parallel to the shear vector, as the northeastern portion of the subcloud cold dome merged with cold air generated by individual storms that had formed ahead of the line. The intensity of the cells within the squads line diminished as its axis became more parallel to the shear.

Trajectory analyses based on the Doppler-derived wind field show that three-dimensional airflow is crucial to the maintenance of the squall line. Boundary-layer air directly ahead of the strongest reflectivity centers fed the associated updrafts while air on their flanks rose slightly, was cooled by evaporation of rain, and then descended to become the primary source of air in the subcloud cold dome. In contrast to typical midlatitude squall lines, there was no evidence of organized rear-to-front system-relative airflow in the subcloud air. This is explained in terms of the initial front-to-rear momentum of the cold-dome source air, with frictional effects also playing a role for air near the surface. Since the ground is traveling rearward relative to the storm, frictional effects oppose the pressure gradient ahead of the cold-dome pressure maximum and keep the near-surface air moving rearward throughout the cold dome. Only a small fraction of the subcloud air originated at midcloud levels, probably because evaporation above cloud base was inhibited by high relative humidities in the environment and because comparatively weak updrafts produced only modest amounts of condensate for water loading.

The persistence of squall-line elements is discussed in light of (a) their resemblance to supercells as represented in numerical simulations, and (b) recent theories involving the balance of vorticity between vertical shear in the low-level environment and the cold dome in the subcloud layer. The squall line is representative of that part of the spectrum of mesoscale convective systems that does not have a rear inflow jet, does not produce a trailing stratiform precipitation region, and does not rely upon penetrative downdrafts to sustain the air mass within the subcloud cold dome.

Abstract

Objective numerical techniques are applied in analyzing constant altitude aircraft measurements obtained from coordinated research flights in thunderstorm inflow regions. The approach combines meteorological and flight track data from dual or single aircraft missions in a common frame of reference and transforms the observations from original analogue format to horizontal two-dimensional Cartesian coordinates. Operational procedures guiding the data collection, intercomparison techniques for refining instrument calibrations and corrections for aircraft navigation errors are all considered.

Results of the interpolations are judged in the context of the storms' associated radar echo features. Primary applications include calculation of water vapor influx in cloud base updrafts. Evidence indicates that the fullest exploitation of the inflow mapping will come through combining kinematic fields observed concurrently by aircraft and multiple Doppler radars.

Abstract

The initiation of deep convection through forcing along boundary layer convergence lines is examined using observations from the Convection and Precipitation/Electrification Experiment conducted in east-central Florida during the summer of 1991. The study is concerned with the evolution and interaction of two converging air masses that were initially separated by an intervening boundary layer characterized by neutral stability and horizontal convective rolls. As anticipated, major thunderstorm erupt when the east coast breeze eventually collides with thunderstorm outflows from the west, but unexpected convection takes place prior to their merger along a well-defined confluence zone associated with a persistent quasi-stationary roll vortex signature. Analyses using wavelet transforms confirm that linear boundary layer reflectivity features are strongly correlated with radial convergence associated with roll vortices. In this study, complementary interactions between roll vortex convergence lines and the sea-breeze front are not sufficient to trigger deep convection. However, organized convergence along the eastward-spreading thunderstorm outflows did interact periodically with roll vortex convergence maxima to initiate a series of new storms.

Results from two-dimensional numerical model simulations replicate many of the observed boundary layer features. Surface heating produces circulations similar to sea-breeze frontal zones that appear near the coastlines and progress steadily toward each other as the interior boundary layer deepens. Vertical velocity maxima develop over the associated convergence zones, but weaker periodic maxima also occur within the interior air mass at intervals similar to the spacing of observed horizontal roll vortices. As the boundary layer deepens, a layer immediately above it cools, confirming organized large-scale ascent within the interior air mass. When surface heating is removed, circulation associated with this large-scale ascent collapses to a near-steady state where the width of the remaining prominent updraft is similar to its depth. This results from a balance between momentum advected by large-scale circulations and excess pressure developed at low levels near the center of the interior domain.

Abstract

A detailed analysis is made of the environmental conditions existing on each of the declared hail days during the randomized seeding experiment. From the many soundings available each day, the one most representative of the near-storm environment is carefully selected. This sounding is then used to compute several parameters known to influence hailfall. It is found that two parameters, both indicative of the thermodynamic instability, have a more instable mean value on the seed days than on the control days that in one case is statistically significant at the 10% level. Correcting for this draw would result in reducing the actual ratios of seed to control hail mass found in the primary statistical evaluation of the experiment. However, the reduction would not be sufficient in relation to the very wide 90% confidence limits to affect the statistical conclusions that the ratios were not significantly different from 1.0.

An analysis of the sequences of declared hail days showed that, in spite of the careful experimental design, the random selection process produced an actual partitioning of sequence starts into seed or control such that a sequence this extreme, or more extreme, had a chance of only 3 in 100 of occurring. However, it is not likely that this unexpected draw affected the evaluation of the experiment in any significant way, since it is taken care of indirectly in the analyses of the environmental parameters.